In the oil and gas industry, bores or wells are drilled from surface to access subsurface hydrocarbon-bearing formations. During the drilling operation, it is conventional to circulate drilling fluid down through the drill string and up through the annulus between the string and the bore wall. The drilling fluid carries various materials, including solids, to provide the fluid with desired properties, and also serves to carry the cuttings generated by the drilling operation to surface. The drilling fluid is generally maintained at a pressure sufficient to prevent fluid contained in permeable formations intersected by the bore from passing into the bore. Thus, the drilling fluid is forced against the permeable bore wall under pressure and as a result a residue of fine material is deposited on the bore wall. This residue is known as filter cake and is generally considered to be desirable as the filter cake isolates the formation from the drilling fluid, minimising fluid leak-off to the formation, a poor filter cake may permit deep invasion of the reservoir by the drilling fluid and result in damage to the formation.
Removal of filter cake may be desirable at some stage in the preparation of the well for production, for example in preparation for cementing casing in the bore to ensure good contact and bonding between the cement and the wellbore wall. Such an operation may include the provision of a scratcher mounted on a joint of casing. A typical scratcher includes stiff wire fingers or cable loops extending outwards from a steel band. As the casing string is deployed into the well or bore or during reciprocation and/or rotation during circulation for clean-up or during cementing the scratcher disrupts and disturbs the filter cake. After lining and cementing, the liner may be perforated to allow production to commence.
Alternatively, or in addition, cleaning fluid may be circulated in the bore, the fluid containing “breakers,” such as hydrochloric acid, enzymes or oxidative solutions. However, such an operation typically results only in partial cake removal.
Increasingly, production of formation fluids takes place through bore wall sections which have not been prepared in the conventional manner, by being lined, cemented and then perforated. For example, a completion may now include extended sand screen sections which are located in an extended unlined bore section. In such a situation, the presence of filter cake may have a more significant bearing on production. Indeed, it is often observed that only a very small fraction of the filter cake is removed by conventional methods, while most of the formation still remains covered by the cake. This condition results in only small portions of the flow area contributing to hydrocarbon inflow. Furthermore, once a small fraction of the formation is open to flow, there may be insufficient pressure gradient or differentials to remove the filter cake from other parts of the formation face.
WO 2009/001069 and WO 2009/001073, the disclosures of which are incorporated herein in their entirety, describe arrangements for supporting borehole walls and for applying predetermined stresses to borehole walls. Inflatable chambers are mounted on a base pipe such that inflation of the chambers increases the diameter of the assembly. The chambers may support a sand control element.
WO 2012/066290 and GB 2492193 A, the disclosures of which are incorporated herein in their entirety, describe other arrangements including inflatable chamber to support elements such as sand control screens.